The mixing of two or more fluids for purposes of activating the bonding or sealing properties of the mixture has been well established. This process needs to ensure that the two fluids are accurately metered before being mixed. Typically, for structural adhesives, there is a relatively thick first fluid, which may be referred to as the base material, that is to be mixed with a relatively thin second fluid, which may be referred to as the catalyst. Separate pumps are provided which pump the base material and the catalyst material from larger containers full of such material, into a metering device. The metering may include two, double acting piston cylinder assemblies (e.g., balanced or nondifferential cylinders), one to be filled with the catalyst and another to be filled with the base. These cylinders are designed such that a ratio of their internal volumes containing the catalyst and base are within a range specified for the mixture. The piston inside each of the cylinder assemblies divides the internal volume into front and back portions. The piston moves under pressure of the fluid that is being pumped into one portion, pushing the “metered” amount from the other portion. A four-way valve is also provided for each of the base and catalyst materials, which alternately routes the two portions of each cylinder assembly to a respective input of a mixing chamber. The pumps operate continuously, pumping their respective fluids into the metering cylinder assemblies, while the four-way valves automatically change their positions so that in one half of a cylinder cycle a first portion of each cylinder is being filled with its respective fluid while in the second half of the cycle that filled volume is emptied through the four-way valve (while the second portion is being filled). The output of the mixing chamber serves to dispense the mixture into for example a cartridge that holds the mixture until the mixture is to be applied to the bonding surfaces.